Re: [XEN PATCH v12 2/7] x86/pvh: Allow (un)map_pirq when dom0 is PVH

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Jul 08, 2024 at 07:41:19PM +0800, Jiqian Chen wrote:
> If run Xen with PVH dom0 and hvm domU, hvm will map a pirq for
> a passthrough device by using gsi, see qemu code
> xen_pt_realize->xc_physdev_map_pirq and libxl code
> pci_add_dm_done->xc_physdev_map_pirq. Then xc_physdev_map_pirq
> will call into Xen, but in hvm_physdev_op, PHYSDEVOP_map_pirq
> is not allowed because currd is PVH dom0 and PVH has no
> X86_EMU_USE_PIRQ flag, it will fail at has_pirq check.
> 
> So, allow PHYSDEVOP_map_pirq when dom0 is PVH and also allow
> PHYSDEVOP_unmap_pirq for the removal device path to unmap pirq.
> And add a new check to prevent (un)map when the subject domain
> doesn't have a notion of PIRQ.
> 
> So that the interrupt of a passthrough device can be
> successfully mapped to pirq for domU with a notion of PIRQ
> when dom0 is PVH
> 
> Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
> Signed-off-by: Huang Rui <ray.huang@amd.com>
> Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
> ---
>  xen/arch/x86/hvm/hypercall.c |  6 ++++++
>  xen/arch/x86/physdev.c       | 12 ++++++++++--
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/hypercall.c b/xen/arch/x86/hvm/hypercall.c
> index 0fab670a4871..03ada3c880bd 100644
> --- a/xen/arch/x86/hvm/hypercall.c
> +++ b/xen/arch/x86/hvm/hypercall.c
> @@ -71,8 +71,14 @@ long hvm_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>  
>      switch ( cmd )
>      {
> +        /*
> +        * Only being permitted for management of other domains.
> +        * Further restrictions are enforced in do_physdev_op.
> +        */
>      case PHYSDEVOP_map_pirq:
>      case PHYSDEVOP_unmap_pirq:
> +        break;
> +
>      case PHYSDEVOP_eoi:
>      case PHYSDEVOP_irq_status_query:
>      case PHYSDEVOP_get_free_pirq:
> diff --git a/xen/arch/x86/physdev.c b/xen/arch/x86/physdev.c
> index d6dd622952a9..9f30a8c63a06 100644
> --- a/xen/arch/x86/physdev.c
> +++ b/xen/arch/x86/physdev.c
> @@ -323,7 +323,11 @@ ret_t do_physdev_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
>          if ( !d )
>              break;
>  
> -        ret = physdev_map_pirq(d, map.type, &map.index, &map.pirq, &msi);
> +        /* Only mapping when the subject domain has a notion of PIRQ */
> +        if ( !is_hvm_domain(d) || has_pirq(d) )

I'm afraid this is not true.  It's fine to map interrupts to HVM
domains that don't have XENFEAT_hvm_pirqs enabled.  has_pirq() simply
allow HVM domains to route interrupts from devices (either emulated or
passed through) over event channels.

It might have worked in the past (when using a version of Xen < 4.19)
because XENFEAT_hvm_pirqs was enabled by default for HVM guests.

physdev_map_pirq() will work fine when used against domains that don't
have XENFEAT_hvm_pirqs enabled, and it needs to be kept this way.

I think you want to allow PHYSDEVOP_{,un}map_pirq for HVM domains, but
keep the code in do_physdev_op() as-is.  You will have to check
whether the current paths in do_physdev_op() are not making
assumptions about XENFEAT_hvm_pirqs being enabled when the calling
domain is of HVM type.  I don't think that's the case, but better
check.

Thanks, Roger.